Regardless of the dispatching strategy used to assign up and down hall calls registered from the floors of a building to a group of elevator cars, in the absence of up or down peak traffic conditions, it would be desirable to balance the number of cars serving up demands with the number of cars serving down demands, to achieve an improved distribution of cars throughout a building and hence a lower average waiting time (AWT). The AWT is the industry standard for measuring the efficiency of elevator systems.
Other then in a morning up peak condition, which initiates well known special strategies for quickly returning cars to a traffic entry floor, ie., lobby floor, and in the absence of an evening down peak condition, which also initiates known special strategies for quickly de-populating a building, a balanced distribution of elevator cars will more efficiently serve existing hall calls. When traffic subsides and then picks up, the cars will already be dispersed throughout the building and in a better position to quickly serve new hall calls.
Some dispatching strategies deliberately move the elevator cars to predetermined floors of the building when traffic subsides, ie., during an off peak traffic condition, called "spotting". Spotting is inefficient, as the cars are doing no useful work during their travel to the preselected floors. In addition, they are wasting energy are creating unnecessary wear and tear which increases maintenance costs.
A dispatching strategy which is attractive because of its simplicity, and which would at first appear to be a very efficient strategy, estimates the time of arrival (ETA) of each elevator car at the floor of a specific hall call to be assigned. The hall call assignment is given to the elevator car having the lowest ETA. The call assignments are continuously reevaluated, and when a previously assigned hall call is being reconsidered, reassignment to another car is only made to another car having a lower ETA when the lower ETA is lower by T seconds.
The ETA strategy, however, can lead to car distribution problems. Cars can bunch or cluster and race one another to answer hall calls. This leads to leap frogging and "no-call stops" in which a car stops only to find another car has just arrived to serve the same call.
The present invention relates to improving elevator dispatching strategies in general, and to improving the ETA strategy in particular, and it is an object of the invention to improve car distribution in a building by incorporating the improvement in the assignment process itself, reducing the need to artificially spot cars throughout a building.
The following co-pending applications, which are assigned to the same assignee as the present application, are also directed to improvement of the ETA strategy, with the present invention complementing the methods disclosed therein:
(1) Application Serial No. 168,791, filed March 16, 1988, entitled "Coincident Call Optimization In An Elevator System";
(2) Application Serial No. 168,817, filed March 16, 1988, entitled "Dynamic Assignment Switching In The Dispatching Of Elevator Cars";
(3) Application Serial No. 169,206, filed March 16, 1988, entitled "Method For Using Door Cycle Time In Dispatching Elevator Cars"; and
(4) Application Serial No. 169,210, filed March 16, 1988, entitled "Anti-bunching Method For Dispatching Elevator Cars".
Application Serial No. 168,817 is hereby incorporated into the specification of the present application by reference.